1. Field of the Invention
The present invention relates to an image reading device, particularly to an image reading device employed in a digital copy machine or an image scanner, and an image forming apparatus provided with the image reading device.
2. Description of the Related Art
In recent years, along with the development of light-emitting diodes (hereinafter referred to as LED), the brightness of an LED element has been greatly enhanced. Having advantages such as a long service life, high efficiency, high shock-resistance, monochromatic light emission, etc., the LED is expected to be applied extensively in many illumination-related fields.
For example, the LED is employed in a document illuminating device (a device for illuminating paper), provided in an image reading apparatus like a digital copy machine or an image scanner.
There are proposed various methods for the application of the LED employed in the image reading apparatus. For example, Japanese Patent Application Publication Numbers 2006-67551, 2006-42016, and 2005-241681 disclose that a plurality of LEDs are aligned in parallel with a main scanning direction of a document. In the main scanning direction, light emitted from the plurality of LEDs is diffused by an optical system without being concentrated. In a sub-scanning direction, light emitted from the plurality of LEDs is concentrated by a reflection member, whereby the illuminance of a reading target area on a document placing surface is enhanced.
In addition, Japanese Patent Application Publication Number 2005-311662 discloses that a plurality of LEDs are aligned in parallel with a main scanning direction of a document. In a main scanning direction, light emitted from the plurality of LEDs is diffused by an optical system without being concentrated. In a sub-scanning direction, light emitted from the plurality of LEDs is concentrated by a lens, whereby the illuminance of a reading target area of a document placing surface is enhanced.
Next, it will be explained how the illuminance or illuminance distribution is obtained on the document placing surface when the document is illuminated by the light emitted from the plurality of LEDs.
FIG. 9 shows a schematic configuration of a conventional image reading apparatus. As illustrated in FIG. 9, as to the image reading apparatus employed in the digital copy machine or in the image scanner, an image of the document is obtained by an image pickup element such as a CCD 14 via a readout lens 13.
FIG. 10 is a top view of the document placing surface 15 illustrated in FIG. 9. When an image reading optical system constituting the image reading apparatus is in a fixed state, only image information of an elongated reading target area 10, illustrated in FIG. 10, can be input into the image reading apparatus via the CCD 14. Therefore, via either moving the entire apparatus illustrated in FIG. 9, including an illumination lamp 22, a reflector 21 facing the illumination lamp 22, a plurality of mirrors 16, a lens 13 and the CCD 14, or moving an illumination optical system including the illumination lamp 22 and the reflector 21 in conjunction with the plurality of mirrors 16, the reading target area 10 is moved in a direction of an arrow in FIG. 10. The entire document can be read out via obtaining the image information sequentially by the CCD 14 while the reading target area 10 is being moved in the sub-scanning direction.
At this time, the reading target area 10 illustrated in FIG. 10 is moved quickly. Consequently, the reading time per sheet of the document is shortened, which causes a reduction in an amount of light incident onto the CCD 14 per unit time. Therefore, it is desired to increase the amount of light illuminating the document. Considering this, the methods of confining the light to the sub-scanning direction disclosed in the above-mentioned prior art references are appropriate.
Meanwhile, generally, a uniform illuminance distribution is desirable. FIG. 11 illustrates the relationship between the document placing surface 15 and the CCD 14, when the document placing surface 15 and an image pickup area of the CCD 14 are arranged to be in a conjugate positional relationship in a length direction (the main scanning direction) indicated in FIG. 10. Two examples of an illuminance distribution on the document placing surface 15 are illustrated by a solid line and a dotted line.
In FIG. 11, the solid line represents an illuminance distribution on the document placing surface 15 at a predetermined time, while the dotted line represents an illuminance distribution on the document placing surface 15 when shaking (in the main scanning direction) such as a vibration from outside is undesirably applied to the image reading apparatus at the time. As illustrated by the solid line or the dotted line in FIG. 11, when there is non-uniformity in the illuminance distribution on the document placing surface 15, with respect to a position of high illuminance on the document placing surface 15, a corresponding position on the image pickup area is of high illuminance also. Conversely, with respect to a position of a low illuminance on the document placing surface 15, a corresponding position on the image pickup area is of low illuminance also. If the illuminance distribution on the document placing surface 15 in the main scanning direction is uniform, even if the above-mentioned vibration occurs when the reading target area is moved in the direction of the arrow, indicated in FIG. 10, in order to read the entire document, a density irregularity on a scanned image of the document cannot be generated. However, when the illuminance distribution in the main scanning direction is non-uniform and the above-mentioned vibration occurs, the state illustrated by the solid line may be shifted to the state illustrated by the dotted line. Consequently, the density irregularity is generated partly in the scanned image, which results in deterioration in an image quality.
Therefore, as illustrated in FIG. 12, an illuminance distribution in the main scanning direction over an entire reading target area is desired to be uniform in general. The reason is that when the illuminance distribution is uniform, even if shaking such as the vibration from outside occurs, the illuminance distribution on the image pickup area would not be varied accordingly.
The illuminance distribution in a width direction (sub-scanning direction), as indicated in FIG. 10, is particularly desired to be uniform. However, unfortunately, for example, when the light emitted from a light source is concentrated to the sub-scanning direction, as illustrated in FIGS. 1, 3, and 5 of Japanese Patent Application Publication Number Hei02-22679, a steep illuminance distribution (with a sharp-pointed peak) is formed undesirably on the document placing surface 15, as illustrated by the solid line in FIG. 13. With such a steep illuminance distribution, when the illuminance distribution is instantaneously shifted from a state indicated by the solid line to a state indicated by the dotted line, the amount of light incident on the CCD 14 differs remarkably, as indicated by an arrow of FIG. 13.
Therefore, as illustrated in FIG. 14, in the sub-scanning direction, a uniform illuminance distribution with a certain width (for instance, about 3 mm) is desired. Considering this, a mirror in an oval shape in cross-section illustrated in Japanese Patent Application Publication Number Hei02-22679 is not necessarily the most desirable. Instead, if a cross-section of a mirror is in a curved shape, a parabolic shape is desired. A planar mirror may also be appropriate as long as a light distribution state (illuminance distribution) is uniform.
According to one embodiment of the present invention, since a plurality of point light sources such as an LED light source, an organic EL light source, etc. are employed, which differ from a tubular light source in Japanese Patent Application Publication Number Hei02-22679, when approaching the document placing surface, an illuminance irregularity tends to appear in the main scanning direction (because the respective point light sources are strong in illumination directivity). Therefore, a certain distance needs to be kept between the plurality of point light sources and the document placing surface so as to eliminate such an illuminance irregularity.
In order to miniaturize a document illuminating device while keeping the distance between the above-mentioned plurality of light sources and the document placing surface, the following two characteristics (I) and (II) are desired. Here, FIGS. 15, 16 and 17 illustrate a schematic structure of an image reading apparatus including the plurality of LEDs respectively.
(I) As illustrated in FIGS. 15, 16 and 17, concentration members 17 and 18 are provided only in a light emission direction of an LED 9 (light source). (In general, at the back of the LED 9, a reflection surface is unnecessary because unlike the tubular light source with 360° directivity with respect to an axis thereof, the plurality of LED light sources or organic EL light sources emit the light with a strong directivity in a frontward direction thereof.) Since the reflection surface does not need to be provided at the back of the LED 9, compared with FIG. 1 of Japanese Patent Application Publication Number Hei02-22679, the document illuminating device can be miniaturized in the width direction (sub-scanning direction).(II) As illustrated in FIG. 17, the light-emitting direction of the LED 9 is arranged to be in parallel with a contact glass 12. In this case, (because although the directivity of the LED is strong, the emitted light diffuses gradually.), by keeping a sufficient distance in movement of the LED 9, the illuminance irregularity in the main scanning direction is eliminated and the document illuminating device is thinned in a height direction.
Here, a problem that needs further considering is that when the illumination fails to reach the entire document to be read out, a black shadow is formed in a read out image of the document. For example, when a book, etc. is opened to be placed on the contact glass 12 (platen), a part between opened pages levitates inevitably due to the structure of the book, thereby such a levitating part between pages is angled to the contact glass 12. Because of the existence of such a levitating part, illumination from only one direction can not illuminate all of the opened pages. Consequently, the problem arises that the black shadow undesirably appears in the read out image of the document.
Conventionally, as to a light source including a cold-cathode tube, as disclosed in Japanese Patent Number 4170878, via providing a reflector facing a light source, a reading target area is illuminated from two directions with respect to a plane perpendicular to a movement direction of a document illuminating device and the reading target area.
However, unfortunately, the above-mentioned “black shadow” problem is not considered in any of Japanese Patent Application Publication Numbers 2006-67551, 2006-42016, 2005-311662 and 2005-241681. In addition, as to Japanese Patent Number 4170818, for the sake of energy conservation, the number of a plurality of LEDs needs to be reduced. If the number of the plurality of LEDs is reduced (since an interval between the LEDs increases accordingly), in order to ensure the illuminance distribution in the main scanning direction, a distance between a light source 6 and the document placing surface needs to be kept adequately, as illustrated in FIG. 1 in Japanese Patent Number 4170878. Therefore, since a distance between the document placing surface and a first mirror needs to be kept adequately, an entire illumination optical system may become large in size.
Although an image reading apparatus is configured with the above consideration, as illustrated in FIGS. 15, 16 and 17, due to respective support structures of respective reflectors, a problem arises that the uniformity of the illuminance distribution deteriorates in the sub-scanning direction.
For example, as illustrated in FIG. 15, the support structures of a reflector 19 and a reflector 20 are simplified at the cost of a decline in a supporting strength thereof. Due to an influence such as an external vibration, etc., the positional relationship between the reading target area 10 and the reflector 19 as well as the reflector 20 is varied easily. Consequently, a problem of two adjacent peaks appearing in the illuminance distribution arises, as illustrated in FIG. 16. (FIG. 16 illustrates an example where, compared with FIG. 15, the reflector 19 and the reflector 20 approach the reading target area.)
In order to improve such an above-mentioned situation, the support structures of both the reflector 19 and the reflector 20 need to be strengthened. However, there are some disadvantages such as the document illuminating device being enlarged or increased in weight.